1. Field of the Invention
The present invention relates to differential gear drives for motor vehicles and, more particularly, to an improved differential which employs eccentrically arranged planetary gear pairs for transmitting power from the differential housing to the wheel axles.
2. The Prior Art
Prior art differential gear drives have generally been of three basic types: the standard bevel gear differential, the so-called limited-slip differential, and the lock-up differential.
In the standard differential, engine torque is delivered equally to both wheels so long as the vehicle moves in a straight line and both wheels have traction. Upon cornering or loss of traction to one wheel, however, the differential tends to deliver power to the wheel of least rolling resistance. While permitting the wheels to corner, this characteristic of the standard differential renders it incapable of distinguishing between loss of traction on a slippery surface or because of turning. Hence the standard differential has serious traction disadvantages.
Limited-slip differentials overcome the problems of the standard differential to a certain extent. In a typical construction, a clutch located between the differential housing and the side gears engages to limit movement between the housing and the side gears, thereby forcing both axles to rotate with the housing. This results in the transfer of torque to the stationary or slower spinning wheel. Although representing an improvement over the standard differential, by virtue of the transfer of torque to the wheel with the greater traction, the limited slip differential is not fully satisfactory. For instance, this type differential is susceptible to high wear and noise, increased fuel consumption, increased tire wear, unpredictable lock-up and the like. In addition, limited-slip differentials are limited to a maximum torque bias of two-and-one half to one.
The third basic differential type, the lock-up type, locks up both wheels simultaneously in response to wheel slip. This differential, however, loses traction in turns, where the outer wheel must disengage to travel around the turn. While affording a higher torque bias, e.g. ten to one, than the limited-slip differential, the lock-up differential is subject to unpredictable lock-up and is not considered suitable for general highway use.
More recent efforts to overcome the foregoing drawbacks of the basic differential types have included the Gleason Torsen differential, as described in the February 1984 edition of Popular Science. This differential appears to overcome certain of the disadvantages of earlier differentials, but relies on a relatively complex and costly gearworm wheel design to transfer torque between wheels.
Accordingly, a continuing need exists, and has long existed, for a differential that will solve the problems encountered in differential gear design and yet do so in a simple and inexpensive manner.